1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device.
2. Description of the Prior Art
A technique of forming a non-single crystal semiconductor layer on a glass substrate by reduced pressure CVD or plasma CVD followed by heating the substrate at about 600.degree. C. to thereby crystallize the layer into a polycrystal semiconductor layer has been well-known in the field of manufacture of semiconductor devices. Explaining the process of this thermal crystallization, first the temperature is raised from room temperature (i.e. initial stage), second the temperature is maintained at about 600.degree. C. (i.e. intermediate stage) for a few hours to several tens hours, and finally the temperature is lowered to room temperature (i.e. last stage). If a cheap glass substrate utilized for a large-sized liquid crystal display device and so on is subjected to the thermal crystallization process at about 600.degree. C., since a cheap glass substrate has its strain point at about 600.degree. C., the glass substrate shrinks (that is, the volume of the glass at the last stage becomes smaller than that at the initial stage) and thereby internal stress is caused in a semiconductor layer provided on the substrate. Further, photolithography pattern to be used in the subsequent process is deformed due to the shrink of the glass substrate, so that mask alignment in the further subsequent process becomes difficult to carry out. According to an experiment, interface state in the semiconductor layer formed on the substrate was high due to the internal stress, therefore electrical property of the semiconductor layer was bad. For the above reason, a technique for obtaining on a glass substrate a semiconductor layer of excellent electrical property has been required in the field of manufacture of semiconductor devices.